The Feast of the Most Holy and Undivided Trinity is the last great festival of the Christian year. It was also the last to be accepted by the Church. It was as late as 1334 that Pope John XXII, the second of the Avignon Popes, ordered Trinity Sunday to be celebrated all over Christendom, although by then it was already a popular festival in England because Archbishop Thomas Becket had popularised it nearly two centuries earlier. Our Christian calendar spends the first half of the Church's year, from Advent until Pentecost, gradually unfolding the revelation of God in His Son and then in His Spirit, and now comes Trinity Sunday as the climax of it all.

So why is it that the Trinity is nevertheless supposed to be one of the thorniest of Christian doctrines? And why are clergy rumoured to prefer this Sunday of all Sundays to be on holiday?

Well, as so often, the problem lies not with the truth of the doctrine, but with our way of thinking about and expressing the doctrine. The first Christian disciples were not trained philosophers, and it never occurred to them to expound the Trinity using philosophical language. Theirs was a Hebrew cast of mind, which is essentially dynamic, fluid, in its concepts. They had discovered in Jesus Christ the human face of the God of love. They had been filled with the power of that love when they received the Spirit. They had found that they simply couldn't say all that they meant by the word 'God' until they had said 'Father', 'Son', 'Spirit'. And for them, that was enough.

But as the Gospel, the Good News of this dynamic God of Love, spread through the known world, it did so in the lingua franca of the day, which was not Hebrew (of course), not even Latin, but Greek. Greek was the administrative and trading language of the Roman Empire. Roman culture borrowed constantly, parasitically even, from Greek. But once the Good News began to be expressed in Greek rather than Hebrew, it moved into a different mind-set, a different way of looking at things. The old fluidities which simply traced the movement of love within the God of love had to be redefined and reinterpreted in terms of the philosophical categories of the day. And this work of redefinition and reinterpretation went on for centuries - it still goes on - and we have to accept that what makes sense to one generation of philosophers doesn't necessarily make sense to another. And so it is easy to understand why the formularies hammered out in, say, the third and fourth Christian centuries, do not necessarily commend themselves to later generations. And that's why there are still many Christians who don't go near the Trinity because they cannot get their heads round the notions of 3-in-1 and 1-in-3 - and there is even a group (admittedly a very small group) called the Unitarian Church, which claims to have solved the problem by dispensing with the Trinity altogether.

But there's the rub, you see. Deny, as the Unitarians must, that Jesus is divine, God's Son; deny that the Holy Spirit of God is divine, the Spirit of God - and what have you left? A God who never made himself incarnate in this world, bone of our bone and flesh of our flesh. A Jesus who is simply a dead prophet: a fine teacher, a good man, but dead. A Spirit which (not who) is really no more than the best thoughts and intentions of you and of me. Enough for you? I can't speak for you. But not enough for me.

So it looks as if I - and you too, I hope - must persevere with our Trinitarian faith. And, you know, I really don't think it's as difficult as the philosophers would have us believe. Let me give you an example, just one example amongst many, of how 'Trinity' is recognisable to us all. If you are married and have children, you are yourself a Trinity. Your parents, from the moment of your birth, knew you as son or as daughter - and only as son or as daughter. But when you married, you were known in a different dimension of yourself, as husband or as wife. This was a second expression of your being, different from the first but still the same person. And then, when you had children, they discovered a third dimension of your being: they knew you only as father or as mother. So you became simultaneously three people - daughter, wife, mother, or son, husband, father - but yet you remained one person. Three in One.

And so it is with God, who makes himself known to us in three different ways, three manifestations, three dimensions of his being. The Book of Genesis opens with the Spirit of God moving on the face of the waters. That same Spirit descends upon the disciples at Pentecost. To Moses is granted a glimpse of the glory of the Father, and it is the Father who is proclaimed by the prophets. And Jesus is described by John at the beginning of his Gospel as God's Word, God's expression of himself, made flesh. The Word was made flesh and dwelt among us. Jesus himself tells the disciples that 'he who has seen me has seen the Father'. And still today we can know God in these three different dimensions, these three different relationships, and we are captured by these different expressions of God at different times. Sometimes it is the Fatherhood of God that overwhelms me, the God who is over all and above all and beyond all: the God whom I can only know in mystery and majesty, and worship in wonder and in awe. Sometimes it is God the Son who is with me most powerfully, my friend and my brother, who walks with me and talks with me. And sometimes I am most aware of God the Spirit, God within me, God coming to me as Comforter or enabler or strengthener.

This building, this supremely beautiful Abbey Church, has three dimensions. It has height and it has length and it has breadth. Take one away and you have nothing: the Abbey falls down flat. You cannot have a two-dimensional building. And so it is with God. This is why Christians, uniquely, celebrate the Feast of the Holy and Undivided Trinity, because the Trinity is the heart of our Faith. It is the celebration of God over us, the celebration of God with us and the celebration of God in us. And our prayer today, and every day, should be that God will ever be over and with and in our hearts, our lives and our church, today and always. Amen.